The present invention relates to a shift control system for an automatic transmission which is equipped with first and second transmission assemblies arranged in tandem on a power train and enabled to execute the change and control of engine torque during a shift.
Generally speaking, an automatic transmission is equipped with a reduction gear mechanism and a plurality of frictional engagement means and is enabled to achieve an arbitrary one of gear stages by operating a hydraulic control system to change the engaged states of the frictional engagement means selectively.
In recent years, there has been noted an automatic transmission which is equipped with first and second transmission assemblies to achieve a multiplicity of gear stages by switching the first and second transmission assemblies simultaneously or alternately. For example, an automatic transmission, which is equipped with a first transmission assembly capable of switching forward three stages and a second transmission assembly capable of switching forward two states, can execute forward six speed stages.
For shifting this automatic transmission, the individual frictional engagement means of the first and second transmission assemblies have to be simultaneously engaged or released. In this case, it is necessary to synchronize the switching operations of the individual frictional engagement means properly. If the switching operations of the frictional engagement means go out of synchronism when the automatic transmission is to be upshifted in its entirety, there may arise the following phenomena: the rotational states of the individual rotary components begin to change to a downshift and then to an upshift; one transmission assembly comes into a downshift after the other transmission assembly has completed its upshift; and vice versa, that is to say, the downshift of one transmission assembly is completed prior to the beginning of the upshift of the other transmission assembly. As a result, the shifting feel is deteriorated.
In view of these defects, there has been proposed a technology for ending the upshift of a first transmission assembly and the downshift of a second transmission assembly, as disclosed in Japanese Patent Laid-Open No. 61-112846, for example, by sensing the instant, at which the rotational speed of the rotary components of the first transmission assembly is changed as the first transmission assembly is brought into an inertia phase by the supply of the oil pressure to the first transmission assembly in response to a shifting instruction, and by triggering the reductions of engine torque and the engagement pressure of the second transmission assembly gradually at the sensed instant.
It is, however, the fact at present that the actual shifts are not always ended simultaneously even by the aforementioned shift control. In this case, the shifting characteristics are seriously deteriorated: output shaft torque jumps, as shown in FIG. 8, in case the shift at the first transmission assembly delays; but the output shaft torque drops, as shown in FIG. 9, in case the shift at the second transmission assembly delays.